Plot data from stdin

G'day,

I've recently started using some binaries that print numerical data directly to stdout. During debugging calculations, it is a bit cumbersome to save the data to file, then plot.

I have added the ability for gplot to tell gnuplot to read from stdin. I use the letter "-p" for pipe, since "-s" (for stdin) and "-i" were already taken.

example:

```
mycalc.x | gplot -p using 1:2 w lp
# mycalc.x is a binary that outputs numerical data to stdout
```

Caveats:
- incompatible with [-f FOREXPR], requires the data to be repeated for each loop
- incompatible with [-F FREQUENCY], stdin buffer is flushed after reading

Also note that where [-p] and [::: [FILE]...] are both present, [-p] is ignored.